Japan’s ‘Knee Mortar’


The ‘knee mortar’ was among the most peculiar weapons that the Japanese Army brought with it to battlefields in China and elsewhere in the 1930s and 1940s. This detailed article about the Type 89 Heavy Grenade Discharger is reproduced from Teri’s WWII Japanese Handgun Website, with their kind permission. For much, much more on all aspects of the Japanese Army’s weaponry, and especially its handguns, click here.

One of the most feared weapons in the Japanese arsenal was the Type 89 Heavy Grenade Discharger (hachi-kyu-shiki ju-tekidanto, or more commonly just tekidanto). This weapon received the dangerously unfortunate English nickname of “knee mortar”; the curved base plate was meant to be braced against a solid foundation (the ground, a log, etc.), NOT one’s knee, as the recoil would result in broken bones or worse. The Type 89 covered the gap between the range a hand grenade could be thrown and the range of a true mortar. Because of its small size (less than 60cm, or 24” long) and light weight (about 4.7 kg, or 10.5 lbs.), it was easily portable. In a standard Japanese infantry division, a regiment of 3,843 men had about 108 of these, or one per 36 men, according to the US Army Handbook on Japanese Military Forces, October 1, 1944, p. 22. By comparison, the regiment would have had 112 light machine guns and 36 heavy machine guns. The Type 89 was extremely accurate in the hands of a skilled operator. The Japanese made about 120,000 of them. The “89” in Type 89 refers to their introduction in the Japanese year 2589, i.e. 1929.

Here is a front view. The line on the barrel was for aiming purposes. The knob just below the barrel was for range adjustment (more on that below), the trigger mechanism is the lever with the triangular loop in the middle, and the spring at the base was actually a frame for a canvas cover to keep dirt out of the mechanism. A lanyard or leather tab was often attached to the double-action trigger mechanism to make it easier to pull.


Here is a back view. There is a cross-sectional view of the mechanism in the Nakata & Nelson book Imperial Japanese Army and Navy Uniforms & Equipment, New Revised Edition, p. 31.


The serial number is at the base of the barrel. This number is on all major parts. Below the serial number is a string of symbols. The first three say Sho (short for Showa) 14, indicating manufacture in the 14th year of the reign of the Showa Emperor (i.e. Hirohito). This translates to 1939. The next symbol, which looks like a lop-sided 8 in a circle, is the mark of Nagoya Arsenal, under whose supervision it was made. The third symbol is the character Nan/Nam, which literally means “south”. It was the corporate logo of Chuo Kogyo, Japan’s largest war-time private manufacturer of small arms, formed in 1937 by a merger of Lt. Gen. (ret.) Kijiro Nambu’s company with two other private firms. In other words, this one was made by Chuo Kogyo under Nagoya Arsenal supervision. The character at the far right is a bit hard to see in this shot, but it is the kanji to as in Tokyo, which was an inspection mark used by Chuo Kogyo.


This shot shows the range adjustment knob (the knurled cylinder on top). As you turn this knob, it works through gears to move the range adjustment screw up and down. Note that there are two range adjustment scales, one on each side of the slit through which you can see the adjustment screw. The one on the left side (bottom of the photo) is for the Type 89 round, which was purpose-made for such weapons (a deactivated one is shown below). That scale goes from 120 to 650 meters. The one on the right side (top of the photo) is for use with a Type 91 multi-purpose (hand/rifle/mortar) grenade with a booster attached, and goes from 40 to 190 meters.


 Here is a look down the barrel, which is rifled. The calibre is 50mm. The little post you see in the middle has a dual function. It holds the firing pin in the centre, but also serves as an adjustable platform that is part of the range-adjustment mechanism. When the knob referred to in the above photo is screwed to the minimum range position, this little post/platform is very high in the barrel, and only allows the round to drop in about 60mm, or 2.5” (the round sticks out of the barrel at this position—see below). When the range is set to maximum, the post/platform is down at the bottom and the round will drop in about 250mm (10 inches). This is the only range adjustment; the weapon is always held at a 45 degree angle to the ground, and some even had a spirit level sight to let you know when you were at exactly 45 degrees. The way this works is that when it is set at minimum range, there is plenty of room for the gases to expand so they don’t develop very high pressure, and as a result the projectile leaves at a very low speed and doesn’t go very far. At the maximum range setting the pressure is much higher and imparts up a lot more velocity to the projectile, so it travels a lot further. Even at the maximum range setting, however, the muzzle velocity is very slow, well below the limit that defines a firearm in Canada. For example, at maximum range of 650 meters, it takes 13 seconds for the round to reach its target! For that reason Type 89s are not even considered firearms in Canada. By contrast, in the USA, any that have not been de-activated are subject to a lot of red tape. This is a rare situation where Canadian gun laws make more sense than American ones; you can’t get live ammunition for these, so why bother to restrict them? If you did try to duplicate the original rounds, it would already be illegal under regulations on explosives. Some people in the US do make dummy plastic rounds and fire them with a shotgun primer and some black powder just for fun.


The full serial number is on major parts lke the base plate (foot) and the column (there is a lot of deep pitting on the baseplate, but the number is still visible).



Smaller parts like the range adjustment knob have the last three digits of the serial number.


This unit came with a very rare accessory, i.e. this leather barrel cap. As you can see, it is in rather precarious condition. It is obvious it was on this unit a long time, as there is pitting around the outside of the barrel that exactly corresponds to the area covered by the cap (the leather would absorb any moisture and hold it against the metal, eventually causing corrosion). The strap around the bottom of the cap was to tighten it so it would stay on. My specimen of this rare accessory appears in a photo on page 40 of the book The Knee Mortars of Japan 1921-1945 (see book description below). Other accessories included a canvas carrying case and a canvas tool and spare parts kit. I don’t have any of these, but there are some excellent photos in the Sugawa book, Nippon no gunyoju to sogu (“Japan’s Military Guns and Equipment”), pp. 50-51.


 Here is the main round used. It is often called a Type 89 mortar shell, but a Japanese book I have calls it the Type 88. I think Type 89 is correct, but the fuze is a Type 88. The string was used to pull out the safey pin to activate the round, which has a contact fuze (the little thing that sticks out of the top). The basic round was black, with stripes in colours that identified exactly what type it was. This round is pretty well worn, but you can see some of the red at the top and slight remnants of a yellow stripe around the middle. I think the red means Army (vs. Navy) and the yellow designates it as a high explosive round. The brown band around the bottom is actually tarnished copper, which served a special purpose described below. The rounds were carried in a pouch that held four of them (there is a photo in the Sugawa book referred to above). This round is covered in some detail in the Department of the Army Technical Manual TM 901985-5, Japanese Explosive Ordnance (Army Ammunition, Navy Ammunition), pp. 372-376).


The markings on the top of this round indicate it was made in Showa 18, 9th month, i.e. September, 1943.


 The two marks on the far left are the kanji character saka (as in Osaka) and the crossed cannon mark of Osaka Arsenal.


 The body of the round (the middle part that held the explosive) alo has the crossed cannon mark at the far left and the date, read from right to left, 17 in kanji and 11 in standard numerals. This probably indicates that part was made in November, 1942.


The copper band has the number 3074 stamped on it.


Here is a view of the bottom. The holes were to allow gases from the firing of the propellant to escape and drive the round up and out the barrel. The primer was in the centre.


 Here is the round disassembled into the base (bottom), which held the propellant, the body (middle), which held the explosive, and the top, which held the fuze. This round has been de-activated, so it is completely inert, with no explosive or propellant.


Here is the inside of the base. It had an ingenious design to solve the dilemma of how to get a round small enough in diameter to drop down the muzzle freely to also be large enough to engage the rifling. You can see that not only are there holes in the bottom to allow gases to escape out and drive the round upwards, but also holes around the outside edge of the base. You can also see the reddish copper expansion band around the outside. When the primer was struck, it ignited the propellant. The gases from combustion pushed through the holes in the bottom to propel the round out of the weapon, but also outward through the holes around the rim to expand the copper band. The copper band then provided a seal and engaged the rifling to give the round an accuracy-enhancing spin like any rifled weapon. Inside this part there was originally a copper cup to keep the propellant dry, but it is missing. You can also check out the great photos at this link Japanese 50mm Knee Mortar – Inert-Ord.net. For details on the Type 91 grenade and how it was used in the Type 89, please see Japanese Type 91 Grenade – Hand, Rifle or Mortar – Inert-Ord.Net. If you are interested in ordnance in general, check out the rest of the inert-ord.net site, too, as it has lots of great stuff from many countries and was put together by a very knowledgeable collector.


As noted above, when the weapon is set to minimum range, the round protrudes from the barrel. This shot also shows the aiming line more clearly. This was often highlighted with paint to make it easier to see.


I have a manual on shooting techniques for all the standard Japanese weapons, and it has a section on how to shoot the Type 89. The manual is called Shohei shageki kyohan, “All Arms Guide to Shooting”, volume 2, and is dated November 7, 1939. I will translate the text eventually, but as you can see, it basically explains from different angles what to do to shoot the weapon correctly. The first diagram shows a side view of prone shooting. Note the emphasis on holding the weapon at a 45 degree angle to the ground.


Here is a front view of prone shooting.


Here is a side view of shooting from the kneeling position, again showing the 45 degree angle.


Finally, a front view of shooting from a kneeling position.


I also have two manuals that are solely on the knee mortar. This first one is just slightly larger than the typical small black pocket manual format. It is 92mm by 125mm (3-5/8″ X 4-15/16″) and has 90 pages. The date in the back is May, Showa 14 (1939). The title in light golden colour on the front is teki-dan-to-hik-kei, “Manual on the grenade discharger”. The little sticker in the upper right corner is a sort of ownership stamp, it seems. There is very faint green printing on it with numbers penned in with dark blue or black ink. The far right column says Sho-wa 17  5 gatsu (May, Showa 17, i.e. 1942). The middle column says kyo-ka-sho (permission stamp). The column on the left says dai-ni-chu-tai-cho (commander of the second company). The lower left corner of the tag has a red signature stamp on it but it is too faint and blurred to read (there is a name stamp inside the back with the name Okayasu, but I can’t tell if this is the same name). It is mostly text with the same sort of diagrams as above. There is one cross-sectional diagram at the back, but my other manual has a larger and clearer version of the same diagram, so I have reproduced it below. The title is repeated on the spine. The back is plain black.


My other manual is much larger, 148mm by 221mm (5-7/8″ X 8-11/16″). It has 78 pages and is dated June, Showa 13 (1938). the content is much different, with a lot of very technical close-up diagrams. The cover has the title in large characters in the column in the centre: teki-dan-to-tori-atsukai-jo-no-san-ko (Reference on the Handling of Knee Mortars). In the upper right corner it says it was compiled by the Army Infantry School and has a preface by the head of the school, Lt. Gen. (Chusho) Toshio Miyake (the Japanese Wikipedia site on the school says he was its head from August 2, 1937 to June 25, 1938). The lower left says riku-gun-ho-hei-gak-ko-sho-ko-shu-kai-sho. “Army Infantry School Commissioned Officer Assembly Hall”. The title and this last line are repeated on the spine.


The back just has a handwritten note Ichi-chu-tai, ichi-han, i-uchi (First company, first squad, Iuchi, the latter being a family name).


This cross sectional diagram shows the screw mechanism that adjusts for range. It is found on page 4.


 If you want to know everything there is to know about the Type 89 knee mortar and its predecessor, the Type 10, here’s the book you need to get. It just came out in mid-2009 and is available through Banzai. 



Categories: War


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